A chink in somebody’s armour
Definition 1 of 1
Pro Tip 1/3Figurative Use
The phrase is a metaphor from medieval armor, meaning it's not about actual armor but a person's weak point. 
Despite his experience, not adapting to new technology was a chink in his armour.
Pro Tip 2/3Negative Connotation
This phrase often implies a weakness that could be exploited by others. 
Her short temper is a chink in her armour that her opponents use to their advantage.
Pro Tip 3/3Not Always Obvious
The weakness described by the phrase may not be apparent at first and could require insight to identify. 
He seemed invincible until we discovered a chink in his armour – his fear of public speaking.